The present invention is an improvement on the method and apparatus disclosed by the U.S. Scharf et al. U.S. Pat. No. 3,657,793, dated Apr. 25, 1972, and which is hereby incorporated as a reference forming part of the present disclosure.
Precisely stated, the patented method and apparatus concerns the sizing of nuclear fuel rod cladding tubes to bring their insides to diameters substantially precisely fitting the fuel pellets charged in them, for the purpose of reducing to a minimum the play or spacing between the fuel pellets and the insides of the cladding tubes. If this play or spacing is too great, when the fuel rods are in service in a reactor and under the external pressure of a pressurized coolant, the cladding tubes can collapse with the formation of folds weakening the cladding tubes and possibly resulting in a leaky fuel rod. According to the Scharf et al. patent, the play or spacing between the fuel pellets and the insides of the cladding tubes, is reduced to a maximum of 30 .mu., for example.
This is done by the operation of an apparatus comprising an annular pressure chamber having a passage surrounded by the chamber and through which a short length of a cladding tube may be positioned with at least this length containing an internal support having substantially the diameter of the pellets, the support being either the pellets themselves charged into the tube, or a mandrel. A non-metallic elastically-deformable annular seal is inside the chamber for surrounding and sealing the length relative to the pressure chamber, and a high-pressure hydraulic pump is connected to the chamber, this pump having a cylinder with a solid-walled or valveless working portion connected to the chamber, and a reciprocating piston in this working portion, and the pump is provided with an electric solenoid actuator for the piston, the patent also suggesting a pump with appropriate valves although not further disclosing such a pump.
The patented apparatus also includes a means for pushing the cladding tube length-by-length or step-by-step through the hydraulic pressure chamber, this means being timed to the operation of the pump's piston so that each time that piston is effecting a return stroke, a new length of cladding tube is pushed into the pressure zone of the pressure chamber, the cladding tube then being briefly stationary while the pump's piston is forced through a forward stroke, the hydraulic pressure in the chamber compressing the cladding tube against the fuel pellets or mandrel on its inside, and thereby sizing the tube to the precise diameter required to provide the 30 .mu. tolerance maximum considered permissible.
This patented method and apparatus has proved to operate well technically. Both the cladding tube and the fuel pellets are cylindrical, and fuel pellet cladding tubes are ordinarily made of stainless steel, Zircaloy or other metal or alloy having adequate ductility so that under the hydraulic pressure transmitted to the cladding tube through the elastic seal of the patented apparatus, the tube yields and plastically deforms to permit the desired sizing.
When the sizing is done with the fuel pellets on the inside of the cladding tube, the degree of hydraulic pressure applied must be controlled with precision to effect the desired sizing without unduly compressing the fuel pellets themselves. It has proven difficult to provide such precision.
Furthermore, although it would seem that with the tube compressed by hydraulic pressure, that the pressure should be uniform throughout each length progressively sized in the tube's step-by-step progress through the pressure chamber, but under fuel-rod production operating conditions, the cladding tube receives a slightly crowned deformation. This condition must be compensated for by having the successive compression zones overlap each other to an extent undesirably slowing down production rates.